Viruses belonging to the human Caliciviridae, such as Norovirus (previously called the Norwalk virus) and Sapovirus (previously called the Sapporo virus), cause acute enterogastritis in humans. In most cases, stool specimens are used to detect these viruses. In general, ELISA or a similar assay performed on stool specimens is problematic in that not only is its detection sensitivity low, but it also accompanies a lot of non-specific reactions. Therefore, it has been difficult to elucidate results of the assay. In order to improve detection sensitivity, there is a conventional means useful for detecting an antibody that exhibits high reactivity with the antigen. However, such an antibody is too hard to prepare. Incidentally, in order to prevent non-specific reactions which might occur when a stool specimen is employed, a variety of preliminary treatment methods have been performed. Examples of such methods include centrifugation of a specimen to precipitate nonspecific-reaction-related factors for removal and removal of lipids by use of an organic solvent, etc. However, these methods are cumbersome and requires a special apparatus; i.e., a centrifuge. There is another method in which a specimen is suspended in a buffer to which a surfactant added. This method is problematic in that, if non-specific reactions are completely eliminated, its specific reaction weakens whereas if reduction in the incident of specific reactions is prevented, non-specific reactions cannot be avoided completely.